1. Field
The present invention generally relates to wireless cellular communication systems, and more specifically to a wireless communication device and method for selecting a communication cell based on a preference order rather than just signal strength.
2. Background
Wireless communication systems are typically divided into coverage zones having distinct communication frequency channels in each adjacent coverage zones. A wireless communication system, typically identified by a system identifier (SID), may have multiple overlapping or non-overlapping coverage zones to provide communication coverage to wireless mobile devices. Each coverage zone may be defined by a network identifier (NID). One or more communication cells (e.g. antenna towers or base stations) may be associated with each coverage zone or network. As a wireless mobile device travels from one coverage zone to another, it communicates with different communication cells on the same or different communication frequency channels. In a Code Division Multiple-Access (CDMA) communication system, communication cells within a coverage zone may communicate on the same frequency channel.
CDMA is a spread-spectrum communication protocol that utilizes orthogonally coded signals occupying the same spectral bandwidth (e.g., 1.25 MHz). A CDMA signal is spread by one of N (e.g. N=64) orthogonal codes (e.g., Walsch codes or “spreading codes”) that spread the signal over a bandwidth range (e.g., approximately 1.25 MHz). A spreading code is used by each communication cell of a network to transmit signals that are statistically uncorrelated, and therefore separable and non-interfering. For a particular frequency channel, a spreading code is identical for all communication cells (e.g., base stations or towers), except that each communication cell has a different phase-delayed version of the same spreading code. This is usually represented as a time shift or offset measured in chips (e.g., a “chip” may be approximately 0.8 microseconds). This time offset in the spreading code is what uniquely identifies each communication cell (e.g., tower or base station). The pilot channel (spreading code 0) is an unmodified version of the spreading code. It is the pilot channel offset that is used by a mobile device (e.g., mobile phone) to identify a communication cell, distinguish it from other cells, and thereby communicate with the proper cell.
The pilot channel time shift or offset is typically expressed as a “PN offset” reference to absolute time (e.g., PN0). The spreading code sequence repeats periodically (e.g., every 2 seconds). Therefore, PN0 aligns with the beginning of the spreading code period, PN1 is advanced by one time offset (e.g., by 64 chips), PN2 is advanced by two time offsets (e.g., by 128 chips), and so on. The term “PN” stands for “pseudo noise,” which has its origins in spread spectrum theory. There may be up to K (e.g., K=512) unique PN offsets available to network operators.
The PNs used by a particular network operator are typically confined to integer multiples of a PN increment. For example, a PN increment of 3 means that PN0, PN3, PN6, PN9, may be assigned to cells in the network. Each CDMA system operator selects a PN increment value based primarily on its communication cell density. A PN increment of 3 provides more PN time shifts than a PN6 since the total number of unique PN offsets is fixed. PN values may be reused in the same network provided the cells are located at a significant distance from one another and their signals do not interfere with each other. To identify a communication cell, a receiving mobile device measures the time offset (from PN0) of the spreading code of a perceived pilot signal.
Typically, when a wireless mobile device is first switched On, its receiver scans a particular communication frequency channel for pilot signals at different PN offset (e.g. from one or more communication cells). The PN offset having the strongest pilot signal strength is usually selected for communication.
Currently, wireless service providers or carriers are assigned or allocated one or more communication frequency channels and PN offsets in each coverage zone in which they operate. Use of the same communication frequency channel by different wireless service providers in adjacent coverage zones is avoided to prevent interference. Wireless service providers spend significant resources in adjusting their cell antennas (e.g. on base stations) to provide coverage to their subscribers while trying to avoid interference with other carriers utilizing the same frequency channels in nearby areas or coverage zones.
For example, because of the nature of RF propagation, pilot signals on a particular frequency from wireless service provider X in coverage zone A may be stronger in parts of an adjacent coverage zone B than the pilot signals same frequency of wireless service provider Y assigned to coverage zone B. A mobile device in coverage zone B, that is a customer of wireless service provider Y, may pickup the stronger pilot signals from wireless service provider X and communicates through wireless service provider X. This may cause the wireless user to incur unwanted roaming charges. Alternatively, wireless service provider X may deny service to customers of wireless service provider Y, thereby preventing the customer from obtaining a communication link even though he/she is located within the coverage zone of wireless service provider Y.
The use of the same frequency is also a problem where a private wireless network is deployed within a larger public wireless network or overlaps a public wireless network. To maximize the use of their spectrum, the public and/or private operators may use the same communication frequency for the public and private networks. Because mobile devices currently communicate through a communication cell associated with the PN offset having the strongest pilot signal on a particular communication frequency, subscribers of the private network may end up communicating through the public network cells and vice versa.
Thus, a way is needed to intelligently identify and select communication cells operating on the same communication frequency channel in overlapping, adjacent, or co-extensive coverage zones while keeping mobile devices from roaming, unnecessarily, to another provider's network.